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2. CHEMISTRY AND ANALYTICAL METHODS
2.1 Physical and chemical properties
Carbon monoxide (CO) is a tasteless, odourless, colourless, non-corrosive and quite stable diatomic molecule that exists as a gas in theEarths atmosphere. Radiation in the visible and near-ultraviolet (UV)regions of the electromagnetic spectrum is not absorbed by carbonmonoxide, although the molecule does have weak absorption bandsbetween 125 and 155 nm. Carbon monoxide absorbs radiation in theinfrared region corresponding to the vibrational excitation of its elec-tronic ground state. It has a low electric dipole moment (0.10 debye),short interatomic distance (0.123 nm) and high heat of formationfrom atoms or bond strength (2072 kJ/mol). These observationssuggest that the molecule is a resonance hybrid of three structures(Perry et al., 1977), all of which contribute nearly equally to the nor-mal ground state. General physical properties of carbon monoxide aregiven in Table 1.
2.2 Methods for measuring carbon monoxide in ambientair
Because of the low levels of carbon monoxide in ambient air,methods for its measurement require skilled personnel and sophisti-cated analytical equipment. The principles of the methodology havebeen described by Smith & Nelson (1973). A sample introductionsystem is used, consisting of a sampling probe, an intake manifold,tubing and air movers. This system is needed to collect the air samplefrom the atmosphere and to transport it to the analyser withoutaltering the original concentration. It may also be used to introduceknown gas concentrations to periodically check the reliability of theanalyser output. Construction materials for the sampling probe, intakemanifold and tubing should be tested to demonstrate that the testatmosphere composition or concentration is not altered significantly.The sample introduction system should be constructed so that itpresents no pressure drop to the analyser. At low flow and lowconcentrations, such operation may require validation.
Chemistry and Analytical Methods
Table 1. Physical properties of carbon monoxidea
Molecular weight 28.01
Critical point !140 C at 3495.7 kPa
Melting point !199 C
Boiling point !191.5 C
Densityat 0 C, 101.3 kPaat 25 C, 101.3 kPa
1.250 g/litre1.145 g/litre
Specific gravity relative to air 0.967
Solubility in waterb
at 0 Cat 20 Cat 25 C
3.54 ml/100 ml (44.3 ppmm)c
2.32 ml/100 ml (29.0 ppmm)c
2.14 ml/100 ml (26.8 ppmm)c
Explosive limits in air 12.574.2%
Fundamental vibration transition 2143.3 cm1
Conversion factorsat 0 C, 101.3 kPa
at 25 C, 101.3 kPa
1 mg/m3 = 0.800 ppmd
1 ppm = 1.250 mg/m3
1 mg/m3 = 0.873 ppmd
1 ppm = 1.145 mg/m3
a From NRC (1977).b Volume of carbon monoxide is at 0 C, 1 atm (atmospheric pressure at sea level =
101.3 kPa).c Parts per million by mass (ppmm = :g/g).d Parts per million by volume (ppm = mg/litre).
The analyser system consists of the analyser itself and anysample preconditioning components that may be necessary. Samplepreconditioning might require a moisture control system to helpminimize the false-positive response of the analyser (e.g., the non-dispersive infrared [NDIR] analyser) to water vapour and a particulatefilter to help protect the analyser from clogging and possible chemicalinterference due to particulate buildup in the sample lines or analyserinlet. The sample preconditioning system may also include a flowmetering and flow control device to control the sampling rate to theanalyser.
A data recording system is needed to record the output of theanalyser.
EHC 213: Carbon Monoxide
A reference method or equivalent method for air qualitymeasurements is required for acceptance of measurement data. Anequivalent method for monitoring carbon monoxide can be sodesignated when the method is shown to produce results equivalentto those from the approved reference monitoring method based onabsorption of infrared radiation from a non-dispersed beam.
The designated reference methods are automated, continuousmethods utilizing the NDIR technique, which is generally accepted asbeing the most reliable method for the measurement of carbon monox-ide in ambient air. As of January 1988, no equivalent methods thatuse a principle other than NDIR have been designated for measuringcarbon monoxide in ambient air.
There have been several excellent reviews on the measurementof carbon monoxide in the atmosphere (National Air PollutionControl Administration, 1970; Driscoll & Berger, 1971; Leithe, 1971;American Industrial Hygiene Association, 1972; NIOSH, 1972;Verdin, 1973; Stevens & Herget, 1974; Harrison, 1975;Schnakenberg, 1976; NRC, 1977; Repp, 1977; Lodge, 1989; OSHA,1991a; ASTM, 1995; ISO, 1996).
220.127.116.11 Non-dispersive infrared photometry method
Currently, the most commonly used measurement technique isthe type of NDIR method referred to as gas filter correlation (Actonet al., 1973; Burch & Gryvnak, 1974; Ward & Zwick, 1975; Burch etal., 1976; Goldstein et al., 1976; Gryvnak & Burch, 1976a,b; Hergetet al., 1976; Bartle & Hall, 1977; Chaney & McClenny, 1977).
Carbon monoxide has a characteristic infrared absorption near4.6 :m. The absorption of infrared radiation by the carbon monoxidemolecule can therefore be used to measure the concentration of carbonmonoxide in the presence of other gases. The NDIR method is basedon this principle (Feldstein, 1967).
Most commercially available NDIR analysers incorporate a gasfilter to minimize interferences from other gases. They operate atatmospheric pressure, and the most sensitive analysers are able todetect minimum carbon monoxide concentrations of about 0.05 mg/m3
Chemistry and Analytical Methods
(0.044 ppm). Interferences from carbon dioxide and water vapour canbe dealt with so as not to affect the data quality. NDIR analysers withdetectors as designed by Luft (1962) are relatively insensitive to flowrate, require no wet chemicals, are sensitive over wide concentrationranges and have short response times. NDIR analysers of the newergas filter correlation type have overcome zero and span problems andminor problems due to vibrations.
18.104.22.168 Gas chromatography method
A more sensitive method for measuring low background levelsof carbon monoxide is gas chromatography (Porter & Volman, 1962;Feldstein, 1967; Swinnerton et al., 1968; Bruner et al., 1973; Dagnallet al., 1973; Tesarik & Krejci, 1974; Bergman et al., 1975; Smith etal., 1975; ISO, 1989). This technique is an automated, semi-continuous method in which carbon monoxide is separated fromwater, carbon dioxide and hydrocarbons other than methane by astripper column. Carbon monoxide and methane are then separatedon an analytical column, and the carbon monoxide is passed througha catalytic reduction tube, where it is converted to methane. Thecarbon monoxide (converted to methane) passes through a flameionization detector, and the resulting signal is proportional to theconcentration of carbon monoxide in the air. This method has beenused throughout the world. It has no known interferences and can beused to measure levels from 0.03 to 50 mg/m3 (0.026 to 43.7 ppm).These analysers are expensive and require continuous attendance bya highly trained operator to produce valid results. For high levels, auseful technique is catalytic oxidation of the carbon monoxide byHopcalite or other catalysts (Stetter & Blurton, 1976), either withtemperature-rise sensors (Naumann, 1975; Schnakenberg, 1976;Benzie et al., 1977) or with electrochemical sensors (Bay et al., 1972,1974; Bergman et al., 1975; Dempsey et al., 1975; Schnakenberg,1975; Repp, 1977). Numerous other analytical schemes have beenused to measure carbon monoxide in air.
22.214.171.124 Other analysers
Other systems to measure carbon monoxide in ambient airinclude gas chromatography/flame ionization, in which carbon mon-oxide is separated from other trace gases by gas chromatography andcatalytically converted to methane prior to detection; controlled-potential electrochemical analysis, in which carbon monoxide is
EHC 213: Carbon Monoxide
measured by means of the current produced in aqueous solution by itselectro-oxidation by an electro-catalytically active noble metal (theconcentration of carbon monoxide reaching the electrode is controlledby its rate of diffusion through a membrane, which depends on itsconcentration in the sampled atmosphere; Bay et al., 1972, 1974);galvanic cells that can be used to measure atmospheric carbonmonoxide continuously, in the manner described by Hersch (1964,1966); coulometric analysis, which employs a modified Hersch-typecell; mercury replacement, in which mercury vapour formed by thereduction of mercuric oxide by carbon monoxide is detectedphotometrically by its absorption of UV light at 253.7 nm; dual-isotope fluorescence, which utilizes the slight difference in theinfrared spectra of isotopes of carbon monoxide; catalyticcombustion/thermal detection, which is based on measuring thetemperature rise resulting from catalytic oxidation of the carbonmonoxide in the sample air; second-derivative spectrometry, whichutilizes a second-derivative spectrometer to process the transmissionversus wavelength function of an ordinary spectrometer to produce anoutput signal proportional to the second derivative of this function;and Fourier-transform spectroscopy, which is an extremely powerfulinfrared spectroscopic technique.
Intermittent samples may be collected in the field and lateranalysed in the laboratory by the continuous analysing techniquesdescribed above. Sample containers may be rigid (glass cylinders orstainl